The related patent applications describe two-stroke, opposed-piston engines in which pairs of pistons move in opposition to form shaped combustion chambers between their end surfaces. During a compression stroke, two opposed pistons move toward each other in the direction of respective top center locations in the bore of a ported cylinder. As the pistons near the top center locations, charge air is compressed between their end surfaces and fuel is injected through the side of the cylinder into the combustion chamber formed by the end surfaces. The heat of the compressed air ignites the fuel and combustion occurs. In response to combustion, the pistons reverse direction in a power stroke. During the power stroke, the pistons move away from each other toward bottom center locations in the bore. As the pistons reciprocate between top and bottom center locations they open and close ports formed in respective intake and exhaust locations of the cylinder in timed sequences that control the flow of charge air into, and exhaust from, the cylinder.
In some aspects of piston constructions for two-stroke, opposed-piston engines it is desirable to utilize pistons with crowns having contoured end surfaces that interact with swirl and with squish flow from the periphery of the combustion chamber to produce complex, turbulent charge air motion that encourages mixing of air and fuel. However, combustion imposes a heavy thermal load on the piston crowns. The contoured end surfaces create non-uniform thermal profiles that are not suitably cooled by conventional forced cooling configurations, leading to asymmetrical thermal stress, wear, and piston crown fracture. In order to increase piston durability and to contribute to effective thermal management of the engine, it is therefore desirable to provide piston constructions with the capability of cooling the contoured crowns of such pistons.
In some instances, a piston cooling construction for opposed pistons includes an internal annular cooling gallery in each piston through which a liquid coolant (for example, lubricating oil) circulates. See the related, commonly-owned U.S. patent application Ser. No. 13/136,955, published as US 2012/0073526, in this regard. The annular gallery follows the piston's periphery along the under surface of the crown; it is closed except for one or more openings and one or more slots in the gallery floor that respectively admit liquid coolant into and drain liquid coolant from the annular gallery. The dimension of the gallery in the longitudinal dimension of the piston (the height of the gallery) varies between a maximum where the gallery abuts a protruding ridge on the crown end surface and a minimum where the gallery abuts a notch on the end surface through which fuel is injected into the combustion chamber. An opening in the gallery floor provides entry for a jet of liquid coolant transmitted through an open end of the piston skirt. In some instances, these openings are located so as to allow the jets of liquid coolant to strike a portion of the crown under surface lying abutting a ridge on the end surface because the ridge bears a heavy thermal load during engine operation. In some instances, liquid coolant is drained from the annular gallery at about the same level at which the jet enters the gallery. Drained liquid coolant flows into the interior of the piston skirt and then out the open end.
Taking into account oscillation of each of the opposed pistons during high speed operation of the engine and suboptimal drainage through the central gallery, liquid coolant can collect and dwell in a creased portion of an annular gallery under a ridge, creating a standing body of liquid coolant. If a jet is aimed at this portion the standing body of liquid coolant can attenuate the impingement effects of the jet and impair circulation of the liquid within the gallery.
It is desirable for liquid coolant to enter the annular gallery unimpeded and to reach and flow across the crown under surface so as to ensure effective cooling. Further, it is desirable for the liquid coolant to drain unimpeded from the gallery. However, when coolant enters and drains at the same level in the gallery, accumulated coolant in the gallery can disrupt an incoming jet and conversely, an incoming jet can disrupt the coolant moving in the gallery. Either or both of these effects can result in suboptimal circulation through the gallery and muted cooling performance.
It is therefore desirable to improve circulation of liquid coolant in the piston cooling gallery by protecting the incoming jet and reducing or eliminating interference between incoming and effluent streams of liquid coolant in the gallery.